1. Field of the Invention
The present invention relates to a scale plate arrangement in which a scale plate is attached to a machine tool or the like through a scale-plate base for detecting position, velocity and the like of an object.
2. Description of the Prior Art
Hitherto, various scale plate arrangements have been proposed. In case that a scale plate is attached to a machine tool or the like, the scale plate is not directly attached to a machine tool because the scale plate is thin in thickness and thus insufficient in rigidity. In fact, the scale plate is usually attached to a scale-plate base which is made of iron, stainless steel or the like and is higher than the scale plate in rigidity. Then, the scale-plate base having thereon the scale plate is attached to a machine tool.
There have been proposed several conventional ways to attach a scale plate to a scale-plate base. For example, there is provided a proposal in which a scale plate is directly attached to a scale-plate base by means of an adhesive. As to the material of the scale-plate base, it is preferable to use iron or a material which has the same temperature expansion coefficient as that of iron. Such material including iron has magnetism. Therefore, in case that the scale plate is a magnetic scale plate, magnetism of the scale-plate base has a bad influence on a magnetic scale (marks) of the magnetic scale plate. In view of this, there is provided another proposal in Which a non-magnetic material plate made of brass or the like is sandwiched between the scale plate and the scale-plate base so as to prevent the above-mentioned bad influence. In fact, the non-magnetic material plate is attached to the scale-plate base by means of an adhesive, and then the scale plate is attached to the non-magnetic material plate by means of an adhesive. However, the above-mentioned two conventional ways to attach a scale plate to a scale-plate base by means of an adhesive have a drawback. That is, until now, an adhesive which is satisfactory with respect to all the necessary conditions in terms of, for example, resistance against a severe environment, temperature characteristics and curing time has not been available. Such adhesive tends to lower accuracy of the scale plate arrangement. Furthermore, the non-magnetic material plate is generally different from iron in temperature expansion coefficient. Therefore, the temperature change lowers accuracy of the scale plate arrangement having the non-magnetic material plate.
There is provided still another proposal in which a scale plate is attached to a scale-plate base through so-called tension blocks. In fact, the blocks are attached to longitudinal end portions of the scale-plate base with an adhesive, and then longitudinal end portions of the scale plate are attached to the blocks with an adhesive while the scale plate is tensioned. However, vibration or impact tends to make the scale plate surface uneven because only the longitudinal end portions of the scale plate are attached to or supported by the blocks.
There is provided a further proposal in which a scale plate is attached to a scale-plate base through bolts. However, this will increase the production cost of the scale plate arrangement due to the use of the bolts.